A fuel cell is an electrochemical device comprising an electrolyte and respective electrodes (an anode and a cathode) on opposite sides of the electrolyte. Fuel cells may take a variety of different configurations, including planar and tubular. Electrochemical reactions are produced at the electrodes by passing a fuel gas stream across the anode and an oxidant gas stream across the cathode. In the purest form of the reactions, in which the fuel is hydrogen, the outcome is electricity and water, as well as heat since the reactions are exothermic.
In order to produce a useful amount of electricity, a multitude of fuel cells are stacked together in layers, with the stacked fuel cells being electrically connected in series and a load being electrically connected to the ends of the stack. Connecting the layers in series allows for the same current/current density in every layer. A plurality of these stacks may be electrically connected together, but the present invention particularly relates to individual stacks. More than one fuel cell may be provided in each layer of the stack, plural such fuel cells in a layer (at array) being electrically connected in parallel. Adjacent fuel cells, or arrays of fuel cells, in the stack may be separated from each other by one or more of a gas separator, a spacer, a current collector, a seal and possibly other layer components. In a planar fuel cell stack the fuel cells and other stack components are disposed between terminal end plates, which may also provide a manifolding function for the supply and exhaust of the fuel and oxidant gases.
As described above, fuel gas may be supplied to the fuel cells as hydrogen, optionally pre-reformed from hydrocarbon, or, in a suitable high temperature fuel cell stack, such as a solid oxide fuel cell (SOFC) stack or a molten carbonate fuel cell (MCFC) stack, it may alternatively be supplied as a hydrocarbon such as natural gas which is reformed in the stack. The oxidant gas may be pure oxygen, but is more usually air. The gases are commonly supplied to and exhausted from stacked multiple fuel cells or fuel cell arrays through fuel and oxidant supply and exhaust manifolds with fuel and oxidant distribution passages opening to the anodes and cathodes, respectively.
Reforming of hydrocarbons to provide hydrogen as a fuel is an endothermic reaction which, if performed in the stack, is supported by the exothermic oxidation of the fuel on the anodes of the high temperature (≧650° C.) fuel cells. Care is needed to avoid excess cooling from the endothermic reforming reaction.
High efficiency of electricity production in a fuel cell stack requires high fuel utilisation—for example up to 80 to 95% of the fuel supplied to the anodes being oxidised, in the fuel cell reaction. 100% fuel utilisation, or close to it, does not lead to high efficiency because the cell voltage tends to collapse. If there is an inadequate fuel supply to any one fuel cell, that fuel cell can oxidise and cause failure of the cell and, ultimately, of the stack.
Fuel flow to each individual cell or array is controlled by the pressure drop across the fuel supply passages, for example channels in a gas connector, for the cell/array, and hence is determined by the manufacturing tolerance. Manufacturing costs, increase substantially with higher accuracy of the manufacturing process. In order to achieve a fuel utilisation of, say, 90% the flow variation needs to be well below 10%, which would require very high precision and therefore, in one embodiment, expensive gas separators to avoid cell and stack failure. Failure due to over utilisation of individual cells/arrays can only be avoided if flow variation is known and accounted for in the operation. If, for example, the flow variation (due to manufacturing tolerances and thermal effects) is ±20% for a two layer stack and if the cells can be operated at up to 95% fuel utilisation, the overall fuel utilisation of the stack can not exceed 76% in order to avoid damage due to over utilization. The individual fuel utilisation for the above case would be 95% for the low fuel flow layer and 63% for the high fuel flow layer. Failure tolerance of a fuel cell stack due to performance variation or other failure of one fuel cell can be alleviated by the use of arrays of fuel cells in a stack, but invariably an excess of fuel gas must be supplied to the fuel cells in order to minimise the risk of fuel cell oxidation. The outcome of this is that there is unused fuel in the fuel gas exhausted from the fuel cells, that is individual fuel cells and fuel cell arrays are run at relatively low fuel utilisations. The variability of fuel utilisation across different cells/arrays will also lead to a changed thermal profile—higher thermal gradients in cells/arrays that have higher fuel utilisation and vice versa. This can also contribute to stack failures or will at least restrict the maximum fuel utilisation to avoid failures.
In order to improve the overall fuel utilisation, and therefore the efficiency of the fuel cell stack, it has been proposed to recycle the fuel exhaust to the fuel cell stack and mix it with freshly supplied fuel gas. The main advantage of this approach is that the actual fuel utilisation within each cell/array is reduced while the overall utilisation can be very high, depending on the recycling ratio. This also has the advantage of introducing steam, as a product of the fuel cell reaction, to the freshly supplied fuel gas. Steam is necessary for internally reforming hydrocarbons to hydrogen fuel and recycling fuel exhaust means that the freshly supplied fuel gas needs less steam added. See for example WO 2003/019707.
It has also been proposed to improve the overall fuel utilisation of a fuel cell system comprising plural stacks of fuel cells by using the fuel exhaust from one or more stacks as the fuel gas supplied to another stack. One such proposal is in EP 0263052, in which two embodiments are described: a first, in which the fuel gas exhaust from two fuel cell stacks in a first stage is combined and used as the fuel gas supply to a third stack in a second stage; and a second, in which the fuel gas exhaust from one stack (stage 1) is used as the fuel gas supply for a second stack (stage 2) and the fuel gas exhaust from that stack is used as the fuel gas supply for a third stack (stage 3). This proposal is described in U.S. Pat. No. 7,108,929 as involving “the use of a plurality of reactant transfer lines from one stage to the next, which can become complicated and require complicated transfer line assemblies.”
U.S. Pat. No. 7,108,929 is directed to a unitary manifold assembly for use in controlling the flow of reactant gas streams between a plurality of fuel cell stacks, and particularly for combining the fuel gas exhaust from a plurality of fuel cell stacks in a first stage and supplying the combined fuel gas exhaust from the first stage to at least one further fuel cell stack in a second stage.
U.S. Pat. No. 7,482,073 also discloses a multi-stack arrangement with fuel exhaust utilisation from one stack in another stack. In a described embodiment, the fuel exhaust gas from three parallel stacks in a first stage is combined and used as the fuel supply gas to a fourth stack in a second stage. The fuel exhaust gas from the fourth stack is used as the fuel supply gas to a fifth stack in a third stage, and the fuel exhaust gas from that stack is used as the fuel supply gas to a sixth stack in a fourth stage. No reforming of the fuel gas is required in this proposal as it is hermetically sealed and uses hydrogen as the fuel and pure O2 as the oxidant. Water is condensed from the fuel exhaust gas between each stage.
Using the fuel exhaust from one or more fuel cell stacks in a first stage as the fuel supply gas to another stack in a second stage, and so forth, has the advantage of allowing the fuel utilisation in individual fuel cell, stacks to be reduced, and therefore manufacturing tolerances to be eased and cost to be reduced, while giving relatively high overall fuel utilisation. However, the stacks in the different stages are likely to run at different temperatures which, unless they are specifically designed to do so, will impact on their useful lifetimes and their performance. Additionally, more complicated manifolding is required for transferring the fuel exhaust gas from one stack to another. Furthermore, either individual current control for each stack or additional wiring is required, with higher associated costs in either case, and, in the latter case, an increased risk of electrical shorting of the stacks and potentially higher heat losses.
U.S. Pat. No. 6,033,794 also discloses a multi-stage fuel cell system, illustrated in a common pressure vessel, where each stage comprises a stack of fuel cells and the fuel exhaust gas from any one stack is used as the fuel supply gas to a next subsequent stack, but in this case the system is designed to accommodate the different operating temperatures of each stage. This is achieved by each stage being made of a different material, adding considerably to the complexity of the system. The proposal does allow higher fuel utilisation compared to a normal stack, but only at the expense of lower current density in the following stages, and therefore in the following stacks. For example, stage 1 and stage 2 both run at 50% fuel utilisation, leading to 75% overall fuel utilisation, but stage 2 will run at half the current density because it only has 25% of the fuel flow available.
In contrast to the aforementioned prior proposals, in which the fuel exhaust from one stack in a multi-stack arrangement is supplied as fuel to a next subsequent stack, U.S. Pat. No. 5,478,662 (corresponding to EP 0596366 referred to in U.S. Pat. No. 7,482,073 above) describes a fuel cell block or stack comprising sequential multiple stages. Each stage in this proposal comprises plural fuel cells grouped together (optionally with a single fuel cell in the last stage), with some of the fuel exhaust gas from any one stage being used as the fuel supply gas, along with fresh fuel gas, in the next subsequent stage along the stack. The remainder of the fuel exhaust gas from any one stage is discharged to remove water and inert gas components that build up along the fuel gas flow path. This arrangement leads to improved fuel utilisation but only as a result of the discharge of the inerts and the additional fresh fuel gas. Therefore, the amount of fuel available for the electrochemical reaction in the fuel cells in each stage can be kept constant, or at least similar, but the total fuel flow must increase due to the increasing amount of reaction products.
Temperature gradients arise in and between fuel cell stacks due to fuel flow variations across the fuel cells, leading to different cell voltages. In high temperature SOFC or MCFC stacks, temperature gradients can also arise due to different levels of hydrocarbon fuel reforming within the or each stack as well as due to differences in heat loss.
In a high temperature fuel cell system, particularly an SOFC system, such temperature differentials or gradients along and across a fuel cell stack can lead to varied thermal expansion induced stresses along the stack and consequential cracking and failure of the components.
It would be desirable to provide a stack of fuel cells, or fuel cell arrays, which is capable of operating at a high fuel utilisation while alleviating temperature differentials along the stack, and therefore alleviating differential thermal expansion induced stress.